arXiv subjects

Chia-Chin Tsai

Publications and source records attributed to Chia-Chin Tsai.

5 recordsLinked to original sources

arXiv2026

Bosonic Error Correction with Fluxonium

Bosonic quantum error correction (QEC) offers a hardware-efficient route to fault-tolerant quantum computing. To date, however, superconducting circuit implementations of bosonic codes have utilized centimeter-scale 3D microwave cavities controlled by fixed-frequency transmon qubits, with logical lifetimes limited by transmon bit-flip errors. Here, we realize bosonic QEC in a fully planar architecture by pairing a heavy fluxonium, whose $451 \pm 70~μ\mathrm{s}$ bit-flip lifetime exceeds that of any control qubit in previous demonstrations, with an on-chip Archimedean spiral resonator several orders of magnitude smaller in mode volume than prior 3D cavities. We prepare finite-energy Gottesman-Kitaev-Preskill (GKP) states and stabilize them using measurement-free error correction with rapid fluxonium reset, extending the logical lifetime by a factor of $1.59 \pm 0.05$. These results provide the first demonstration of resonator control using a weakly coupled fluxonium and, with it, the first realization of standalone bosonic QEC in a fully planar superconducting circuit architecture.

quant-ph↗

arXiv2026

An All-van-der-Waals Qubit

Advances in solid-state physics, materials science, and device engineering have accelerated the development of superconducting qubits. Among emerging platforms, van der Waals (vdW) materials and their heterostructures are potentially attractive building blocks for quantum devices, yet their realization in qubit architectures remains largely underexplored. Here we report an all-vdW superconducting qubit based on a NbSe$_2$-hBN-NbSe$_2$ junction, in which a thin hBN layer simultaneously provides Josephson coupling and capacitive shunting between two NbSe$_2$ islands, forming a "merged-element" transmon. Temporal characterization using circuit quantum electrodynamics (cQED) techniques yields an average energy-relaxation time $T_{1,\mathrm{avg}} = 55 ~μs$, Hahn-echo coherence time $T_{2\mathrm{E},\mathrm{avg}} = 21 ~μs$, and Ramsey coherence time $T_{2\mathrm{R},\mathrm{avg}} = 1.9~μs$. The relatively low Ramsey time is primarily attributable to an enhanced sensitivity to charge noise consistent with the realized device parameters and not a fundamental limitation. These results show that lumped-element superconducting qubits based on vdW heterostructures can achieve coherence times comparable to those of conventional Al-AlO$_\mathrm{x}$-Al qubits, while offering a reduced device footprint and suppressed stray capacitive coupling.

quant-ph↗

arXiv2022

Deep Learning for Spin-Orbit Torque Characterizations with a Projected Vector Field Magnet

Spin-orbit torque characterizations on magnetic heterostructures with perpendicular anisotropy are demonstrated on a projected vector field magnet via hysteresis loop shift measurement and harmonic Hall measurement with planar Hall correction. Accurate magnetic field calibration of the vector magnet is realized with the help of deep learning models, which are able to capture the nonlinear behavior between the generated magnetic field and the currents applied to the magnet. The trained models can successfully predict the applied current combinations under the circumstances of magnetic field scans, angle scans, and hysteresis loop shift measurements. The validity of the models is further verified, complemented by the comparison of the spin-orbit torque characterization results obtained from the deep-learning-trained vector magnet system with those obtained from a conventional setup comprised of two separated electromagnets. The damping-like spin-orbit torque (DL-SOT) efficiencies (|$ξ_{DL}$|) extracted from the vector magnet and the traditional measurement configuration are consistent, where |$ξ_{DL}$| $\approx$ 0.22 for amorphous W and |$ξ_{DL}$| $\approx$ 0.02 for $α$-W. Our work provides an advanced method to meticulously control a vector magnet and to conveniently perform various spin-orbit torque characterizations.

cond-mat.mes-hall↗

arXiv2022

Toward 100% Spin-Orbit Torque Efficiency with High Spin-Orbital Hall Conductivity Pt-Cr Alloys

5d transition metal Pt is the canonical spin Hall material for efficient generation of spin-orbit torques (SOTs) in Pt/ferromagnetic layer (FM) heterostructures. However, for a long while with tremendous engineering endeavors, the damping-like SOT efficiencies ($ξ_{DL}$) of Pt and Pt alloys have still been limited to $ξ_{DL}$<0.5. Here we present that with proper alloying elements, particularly 3d transition metals V and Cr, a high spin-orbital Hall conductivity ($σ_{SH}{\sim}6.5{\times}10^{5}({\hbar}/2e)Ω^{-1}{\cdot} m^{-1}$) can be developed. Especially for the Cr-doped case, an extremely high $ξ_{DL}{\sim}0.9$ in a Pt$_{0.69}$Cr$_{0.31}$/Co device can be achieved with a moderate Pt$_{0.69}$Cr$_{0.31}$ resistivity of $ρ_{xx}{\sim}133 μΩ{\cdot}cm$. A low critical SOT-driven switching current density of $J_{c}{\sim}3.2{\times}10^{6} A{\cdot}cm^{-2}$ is also demonstrated. The damping constant ($α$) of Pt$_{0.69}$Cr$_{0.31}$/FM structure is also found to be reduced to 0.052 from the pure Pt/FM case of 0.078. The overall high $σ_{SH}$, giant $ξ_{DL}$, moderate $ρ_{xx}$, and reduced $α$ of such a Pt-Cr/FM heterostructure makes it promising for versatile extremely low power consumption SOT memory applications.

cond-mat.mtrl-sci↗

arXiv2021

Anatomy of Type-X Spin-Orbit Torque Switching

Using type-x spin-orbit torque (SOT) switching scheme, in which the easy axis (EA) of the ferromagnetic (FM) layer and the charge current flow direction are collinear, is possible to realize a lower-power-consumption, higher-density, and better-performance SOT magnetoresistive random access memory (SOT-MRAM) as compared to the conventional type-y design. Here, we systematically investigate type-x SOT switching properties by both macrospin and micromagnetic simulations. The out-of-plane external field and anisotropy field dependence of the switching current density ($J_{sw}$) is first examined in the ideal type-x configuration. Next, we study the FM layer canting angle ($ϕ_{EA}$) dependence of $J_{sw}$ through macrospin simulations and experiments, which show a transformation of switching dynamics from type-x to type-y with increasing $ϕ_{EA}$. By further integrating field-like torque (FLT) into the simulated system, we find that a positive FLT can assist type-x SOT switching while a negative one brings about complex dynamics. More crucially, with the existence of a sizable FLT, type-x switching mode results in a lower critical switching current than type-y at current pulse width less than ~ 10 ns, indicating the advantage of employing type-x design for ultrafast switching using materials systems with FLT. Our work provides a thorough examination of type-x SOT scheme with various device/materials parameters, which can be informative for designing next-generation SOT-MRAM.

cond-mat.mes-hall↗